Changes

# One nsMediaDASHStream class is created which manages the monitoring of local capabilities/load and adapts download by switching streams as necessary. Then one nsMediaCache provides data to a single nsWebMDecoder.

'''Description needed'''

# One nsMediaDASHStream as before, but with multiple nsWebMDecoders, one for each encoded stream available on the server. Only one nsWebMDecoder would be used at a time.

'''Description needed'''